1. Radiation and Photometry  Radiation: The process by which energy can be transferred from one body to another by electromagnetic waves in the presence or absence of an intervening medium.  Energy is continually radiated from all substances that are above absolute zero (0 o K).  Emission of electromagnetic energy results in a decrease in the energy level of a molecule and absorption results in an increase.  However, some radiation phenomena do not maintain equilibrium between the thermal motion of molecules and the net gain or loss of radiant energy; such as, emission of energy by radioactive decay and fluorescence of a phosphor under bombardment by electrons.

2.  Radiometry: The science of radiation measurement. l Radiometer: An instrument to measure radiation.  Photometry: The science of visible radiation measurement. l Photometer: An instrument to measure visible radiation.

3. Terms and Units  Radiant energy: The continual emission of energy from the surface of an object. Q l Units: Joules or Watt · sec  Radiant flux: Rate of transfer of energy per unit time from an emitting object. Units: Watts(power term) e = emissivity,  = Stefan-Boltzmann constant A = area of emitting surface

4.  Radiant flux density: Rate of transfer of energy per unit time per unit area from an emitting object or onto a receiving object. In meteorology, it is used as a expression of the thermal effect due to radiation. Units: W/m 2 l Two types: Radiant excitance (leaving a surface) Irradiance (receiving on surface).

5.  Conversion of units: 

6. Luminous Flux  Spectral Flux: radiant flux per unit wavelength interval at wavelength.  Luminous Flux: the spectral flux in the visible portion of the spectrum. The visible energy per unit time.  Spectral forms of radiant flux density are given as M and E.  Within the visible portion of the spectrum, equal amounts of spectral flux of different wavelengths do not produce visual sensations of equal brightness.

7.  Green light (of the same wattage) appears brighter than blue light.  Units used in luminous radiation expressions: l Lumen: Luminous flux issuing from one-sixtieth of a square centimeter of opening of a standard source. l Candela: One lumen/steradian (unit of intensity). The luminous flux per steradian. Equal to 1/683 W/sr.

8.  Illuminance: Luminous flux incident on a unit surface area. l Lux = Lumen/m 2 l Foot candle = Lumen/ft 2 = The illuminance provided by a light source of 1 candela at a distance of 1 foot from the source.

9. Terms  Global Solar Radiation: Radiation from Sun (short wave) incident on a surface. Includes Direct and Diffuse. l Direct Solar: Instrument surface is perpendicular to Sun’s rays. l Diffuse Solar: Scattered. Sun’s direct rays blocked.  Terrestrial Radiation: Radiation from Earth’s surface or atmosphere (longwave).  Total Radiation: Both Solar and Terrestrial, Direct and Diffuse.

10. Radiation Instrument Standards  Primary Standard: l Absolute Pyrheliometer. Measures direct solar radiation. l Meets WMO accuracy (±0.25%) and precision (±0.1% over 1 year) requirements l 4 are maintained by the World Radiation Center in Davos, Switzerland.  Secondary Standard: l Used to calibrate field instruments l Are calibrated against absolute standards.  Network Instruments l Field instruments l May be first class or second class depending on accuracy.

11. Radiation Sensors  Detect radiation by: l Change in temperature of the sensing surface. l Response by photoelectric transducer. l Photochemical response

12. Temperature (thermal) sensors  Following are or have been used to sense the temperature change of the sensor. l Thermocouples l Positive resistance sensor: platinum wire l Thermistors l Bimetallic sensor l Distillation of liquid by heat.

13. Photoelectric Transducer Sensors  Internal Photoelectric Effect; l Photovoltaic Cells: Silicon or Selenium Cells l Semiconductor material is “doped” with a small amount of impurity. These impurities produce diode type devices. One impurity (boron) produces electron “holes,” (the acceptor p-type) and the other (phosphorous) produces extra, loosely held electrons (donor n-type) which move easily when they absorb energy from radiation. l They generate their own current.

14.  When the n-type and p-type are placed together, free electrons on the n-side, rush to the p-side, leaving holes on the n-side.  Not all free electrons fill all the holes. At the junction, a barrier forms, making it harder for electrons on the n-side to move to the p-side.  The +’s and -’s show this barrier.

15.  Eventually, equilibrium is reached and an electric field near the junction separates the two sides. This field acts as a diode, allowing electrons to flow from the p-side to the n-side, but not the other direction.  When photons of light hit the cell, its energy frees electrons. If this occurs near the electric filed near the junction, the field will send the electron to the n- side and an electron hole to the p-side.

16.  An external current path then allows the electron to flow back to its original side, doing work as it does so, to unite with the electron hole.

17. l Photoconductive transducers Voltage is applied across a semiconductor material. When radiation is absorbed by the semiconductor, the resistance to current flow decreases. A measure of the change in current level determines the radiation intensity.

18.  External Photoelectric Effect l Photocells: Radiation causes metal surface to give off electrons which move toward the anode, producing an electric current. l For a given type of material, the wavelength of radiation must be shorter than the threshold frequency in order for current to flow.

19.  Photochemical Devices l Light causes a chemical change in the sensor material which can be detected visually or chemically. l Silver hydride on photographic film is one such material. l Paper emulsed in diazonium salt compounds (Diazo) is another. The diazonium salts chemically change when exposed to light. The salts are neutralized when exposed to ammonia. The darker the paper, the less light received, and the lighter the paper, the more light received. Produces “Blue Line” prints.

20. Radiation Instruments  Pyrheliometer - Direct Solar Beam (or components)  Pyranometer - Global shortwave radiation (or components) on a horizontal plane surface from a hemispheric sky (2  solid angle).  Pyrgeometer - Longwave radiation on an upward or downward facing surface (radiation towards Earth or leaving Earth towards atmosphere and space)

21.  Pyrradiometer - Global radiation (short and longwave) from a hemispheric sky.  Net Pyranometer - Net shortwave radiation.  Net Pyrradiometer - Net shortwave and longwave radiation.

22. Pyrheliometers  Measure Direct Beam Solar Radiation l It is desired to measure all direct beam radiation from the Sun’s disc. l Because R, the distance from the Sun to the Earth varies throughout the year, the solid angle changes and the apparent size of the Sun’s disc changes.

23.  An instrument with a fixed limiting aperture will received radiation from different percentages of the Sun’s disc throughout the year.  The World Meteorological Organization defined a set of design specifications for the geometry of the limiting aperture and placement of sensors in pyrheliometers.

24.  R = radius of limiting aperture  r = radius of sensor receiving surface  d = distance from limiting aperture to sensor

25. Pyrheliometers  Measures direct bean solar radiation l Active Cavity Radiometer An absolute standard (Can define the scale of irradiance without resorting to reference sources) l A WMO Primary Standard l Electric current required to maintain a temperature of 2 cavities at 1 o C above the outer heat sink temperature is measured. l Accuracy ±0.3%

28.  Angstrom Compensation Pyrheliometer l Thermocouples on the back of strips measure the temperature of the strips. l One strip exposed to the Sun. l One strip heated by an electric current until it is the same temperature as the strip exposed to the sun. l When balanced, the energy absorbed by strip from Sun is equal to the energy absorbed by strip from electric current. l Accuracy: ±1.0 to 1.5%

30. Pyranometers  Measure global solar radiation (Direct and Diffuse) l Eppley Black and White Pyranometer Uses 3 black and 3 white wedges (or circular strips) which absorb and reflect incident radiation. Thermocouples within the wedges measure the temperature. White wedges (strips) are cold junctions, black wedges are warm junctions. Difference between the two is a measure of radiation absorbed by black wedges over that of white wedges.

31. l Eppley Precision Spectral Pyranometer Same as Black and White except it has glass filters to limit the wavelengths of radiation incident on the sensor to obtain the intensity in various bands. Measuring reflected solar radiation

32. l Gunn-Bellani Pyranometer (Integrator) Uses 2 bulbs attached to a burette. Space between bulbs is evacuated of air. An inner bulb of blackened copper absorbs radiation. Alcohol evaporates from reservoir in upper part of burette, condenses and enters burette through capillary tube. Amount of alcohol in burette is a measure of radiation received over a period of time. Accuract: ± 2 to 3%

33. l Robitzsch Bimetallic Pyranometer (Pyranograph) Uses two bimetallic strips, one blackened and one white or highly polished. Difference in movement of strips due to temperature change is a measure of radiation absorbed. Large time constant.

34. l Shadow Bands Allow for measuring difuse radiation.

35. Pyrgeometers Measure long-wave radiation from sky if pointed upward or long-wave radiation from Earth if pointed downward. Schott glass filter (usually 3.5  m to 50  m) cover prevents short- wave radiation from entering.

36. Pyrradiometers  Measure total hemispheric shortwave and longwave radiation. l Nova Lynx Pyrradiometer 2 blackened square sensing surfaces each with 90-junction thermopile. One faces upward to measure total hemispheric shortwave radiation. One faces downward to measure total hemispheric longwave radiation. Covered with Lupolene - transparent to 0.3 to 60  m radiation.

37. Radiation Duration measuring Instruments  Typically Solar Radiation Sensors which measure only shortwave radiation.  Campbell-Stokes Sunshine Recorder l Glass ball focuses Suns rays on a treated card. l Rays burn track in card which has time marks on it. l Must be accurately mounted.

38. Meteorological Visibility  Day: The greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a background of fog or sky.  Night: Greatest distance at which lights of moderate intensity can be seen and identified.  Both very subjective.

39.  Meteorological Optical Range l The length of path in the atmosphere required to reduce the luminous flux in a collimated beam (parallel rays) from an incandescent lamp, at a color temperature of 2700 o K to 0.05 (5%) of its original value. l The eye can just recognize patterns of luminance contrast of 5%. l Uses a light source for both day and night visibility. l Can be used with instruments to determine visibility.

40. Photometry  Transmissometer: measures visibility l Projector sends light beam toward a receiver. l Intensity of beam (before transmission) is measured by a sensor (e.g., photocell). l Intensity across known distance drops due to: AbsorptionScattering SpreadingReflection l Change in intensity is a measure of visibility. l Intensity received may also vary due to varying intensity of light source.

42.  Xenon Spark Lamp l Provides best stability Provides modulated light of ~1  s duration l Receiver can be modulated to receive light at same frequency. l This prevents sunlight from affecting the photosensor at the receiver. Produces maximum intensity in range 0.38 - 0.76  m corresponding to light the human eye sees best, ~0.5505  m.

43. ASOS Visibility Sensor  Pulsed xenon flash lamp emits visible light forward into a sampling volume approximately 0.75 cubic feet in size.  Light scattered by particles in the volume is detected by a receiver.  Samples visibility every 30 seconds and determines a 1-minute mean.  A harmonic mean of the last 10 minutes of 1- minute values is used to determine a 10-minute mean to be used in the ASOS report.

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